The present invention relates to a direct injection type internal combustion engine which makes effective use of a swirl of intake air to promote introduction of the air into fuel droplets thereby to improve the combustion efficiency.
In order to improve the combustion efficiency of a reciprocating internal combustion engine thereby to improve the fuel economy and the power output and to reduce the amount of noxious contents in the exhaust gas, it is most important to increase the gas flow rate in the combustion chamber thereby to increase the amount of turbulence. If this is done, the combustion period is shortened.
To increase the amount of the turbulence, a method has been employed whereby the intake swirl is intensified. According to this method, an intake valve is provided with a mechanism for generating a swirl in the intake charge. The valve may, for example, be a shrouded intake valve, a tangential intake port or a helical port which generates a swirl during the intake stroke so that the amount of turbulence is boosted during compression.
A large difficulty concomitant with a small direct injection type Diesel engine of the prior art is the wetting of the wall surface by the injected fuel. The use of the aforementioned method helps to remedy that difficulty.
So long as an intense swirl is present, there arises no problem even if the injected fuel reaches the side wall. When the speed of the engine drops, however, the swirl is weakened, adversely affecting the combustion characteristics. Also, if the engine is driving at low load, the evaporation rate of the fuel is liable to be insufficient because of a drop in the cylinder wall temperature. Also, the emissions of hydrocarbons and white smoke are increased.
On the other hand, generation of an intense swirl is always accompanied by a reduction in the volumetric efficiency. Nevertheless, the following effects of the swirl are expected: (i) an improvement in the dispersion of the fuel droplets, (ii) blowoff of combustion products, and (iii) thermal mixing. Another effect of (iv) suppression of the penetration of the fuel droplets (or of the wall surface from being wetted with the fuel droplets) may also occur.
Moreover, either an internal combustion engine employing a fuel injector in the combustion chamber and an ignition device such as an ignition plug or a direct injection type Diesel engine which is equipped with a direct injection nozzle to increase the compression ratio to ensure auto-ignition of the mixture can employ a method of combining the intake swirl and a squish. In the former, the fuel is injected at a timing of 60 degrees before the TDC in the intake stroke for the case of combustion of a homogeneous mixture. For the case the diffuse combustion is stressed, the fuel is injected at a timing from 60 degrees before TDC to a timing of TDC in the intake stroke. In each case the fuel is directly injected into the combustion chamber so that the sucked air is chilled by the latent heat of evaporation of the fuel and combustion starts at the instant when the air and the fuel are mixed. Compared with a spark ignition type internal combustion engine using a carburetor, there is little danger of knocking. Thus, the compression ratio can be set at a high value.
In such a case, the piston is provided with recesses in the pistons and with intake ports for generating swirls in the suction stroke so that turbulence is generated as aforementioned. Since turbulence is generated immediately before reaching the TDC, the fuel and the air are adequately mixed. However, the strength of the turbulence is insufficient for promoting combustion after reaching the TDC, i.e., after the middle period of the combustion. As a result, combustion in the second half of the combustion period is insufficient, and hence the fuel economy is not sufficient, despite the increased compression ratio. Moreover, since the fuel is directly injected into the combustion chamber, unburned hydrocarbons are liable to be emitted.
In the direct injection type engine, on the other hand, the latter half of the fuel injection cycle is effected when combustion has already started. The compression ignition type direct injection (or Diesel) engine uses a combined method which makes use of both the swirl generated in the suction stroke and the squish generated by providing the piston with a recess, which recess is dish shaped or toroidally shaped. As a result, at the time of forming the mixture, especially at the initial state thereof, the swirl and the intense squish advantageously disperse the fuel. However, after the middle period of combustion, the turbulence may weaken, causing a deterioration in the fuel economy and the emission of black smoke.